All kinds of machine tools can be divided into ordinary precision grade, precision grade and high precision grade according to the precision.
The machine tools of the above three precision grades have corresponding accuracy standards. If the tolerance is 1 for the ordinary grade, the approximate ratio is 1:0.4:0.25. In the design stage, the accuracy of the machine tool is mainly improved from the aspects of the machine tool’s accuracy distribution, components and material selection.
1) Geometric accuracy
Geometric accuracy refers to the accuracy of the shape, mutual position and relative movement of each main component when the machine tool does not move (the machine tool spindle does not rotate or the worktable does not move, etc.) or when the moving speed is low under the condition of no load of the machine tool. Such as the straightness of the guide rail, the radial runout and axial movement of the main shaft, the parallelism or perpendicularity of the main shaft centerline to the moving direction of the slide table, etc. The geometric accuracy directly affects the accuracy of the workpiece, and is the basic index for evaluating the quality of the machine tool. It mainly depends on the structural design, manufacturing and assembly quality.
2) Motion accuracy
Motion accuracy refers to the accuracy of the main parts of the machine tool when they move at the speed of the working state. Such as the rotation accuracy of the high-speed rotary spindle. For high-speed and precise machine tools, motion accuracy is an important indicator for evaluating the quality of machine tools. Movement accuracy and geometric accuracy are different. It is also affected by the movement speed (rotation speed), the gravity of the moving parts, the transmission force and the friction force. It is related to factors such as structural design and manufacturing.
3) Transmission accuracy
The transmission accuracy refers to the coordination and accuracy of the relative movement between the end effectors of the drive system of the index control machine tool. The error in this regard becomes the transmission error of the transmission chain. For example, when the lathe is turning threads, the movement of the tool holder should be equal to the lead of the dry thread for each revolution of the spindle. But in fact, due to the error in the transmission chain between the spindle and the tool holder, the actual shift distance of the tool holder and this error are the transmission error of the thread transmission chain of the lathe. The transmission accuracy is determined by the design of the transmission system, the error of the ideal shift distance, and the manufacturing and assembly accuracy of the transmission parts.
CNC machine tool is the abbreviation of digital control machine tool, which is an automatic machine tool equipped with a program control system. The control system can logically process the program specified by the control code or other symbolic instructions, decode it, represent it with coded numbers, and input it into the numerical control device through the information carrier. After arithmetic processing, various control signals are sent by the numerical control device to control the action of the machine tool, and the parts are automatically processed according to the shape and size required by the drawing.
One is the design of the machine tool. This is the soul of machine tool accuracy.CNC machine tools don’t seem to be anything special, but they are highly accurate and durable. High-precision machine tools have a perfect grasp of design details.
The second is machine tool castings. A machine tool casting is the bone, the key to bear the cutting torque force and disperse the vibration force, and is the basis for the stability of the whole machine. A high-precision machine tool, without good castings, even if it is a spindle screw guide, the accuracy of the whole machine will be gone after a year.
Third, the system determines the key accuracy of the machine tool. Conventional imported systems include Fanuc, Mitsubishi, Siemens, HEIDENHAIN, and Rexroth, domestically produced ones include Guangshu and Huazhong CNC, and most machines have Fanuc. Fanuc systems are widely used in gantry, horizontal, vertical, lathe, sheet metal machine tools, and even robots.
The accuracy of the machine tool refers to the shape, mutual position and relative motion accuracy of the main components of the machine tool, including geometric accuracy, transmission accuracy, motion accuracy, positioning accuracy and accuracy retention.